LCD devices are widely used as display devices for various data-processing devices such as computers and televisions. In particular, TFT LCD devices (hereinafter, also referred to as “TFT-LCD”) become popular, and expansion of the TFT-LCD market is expected. Such a situation creates a demand for much improved image quality.
Although the present description employs the TFT-LCD as an example, the present invention may be applicable to general LCDs such as passive matrix LCDs and plasma address LCDs, in addition to the TFT-LCDs.
The most widely used mode in the TFT-LCDs currently is a mode in which a liquid crystal having positive dielectric anisotropy is horizontally aligned between parallel substrates, namely, the TN (twisted nematic) mode. In a TN LCD device, the alignment direction of LC molecules adjacent to one substrate is twisted by 90° to that of LC molecules adjacent to the other substrate. Such TN LCD devices are now produced at low cost and have been industrially mature, while they are less likely to achieve a higher contrast ratio.
In addition, there are known LCD devices having another mode in which a liquid crystal having negative dielectric anisotropy is aligned vertically to parallel substrates, namely the VA LCD devices. In the VA LCD devices, LC molecules are aligned almost vertically to the surfaces of the substrates when no voltage is applied. Here, the liquid crystal (LC) cell hardly shows birefringence and optical rotation, and light passes through the LC cell while hardly changing in its polarization state. Thus, in the case of the arrangement such that the LC cell is interposed between two polarizers (linearly polarizers) absorption axes of which are orthogonal to each other (hereinafter, also referred to as cross-Nicol polarizers), it is possible to display an almost perfectly black screen when no voltage is applied. When a voltage not lower than a threshold voltage is applied (hereinafter, simply referred to as “the presence of an applied voltage”), the LC molecules are made to be almost parallel to the substrates, the LC cell shows large birefringence, and the LCD device displays a white screen. Thus, such a VA LCD device easily achieves a very high contrast ratio.
The VA LCD devices show asymmetric viewing angle characteristics when LC molecules are all aligned in the same direction in the presence of an applied voltage. In view of this, for example, MVA (multi-domain VA) LCD devices, which are one kind of the VA LCD devices, are now being widely used. According to the MVA LCD devices, the LC molecules in each pixel are aligned in multiple directions by a structurally-modified pixel electrode or an alignment control member such as a protrusion formed in a pixel.
The MVA LCD devices are so designed that an axial azimuth of a polarizer makes an angle of 45° with respect to an alignment azimuth of LC molecules in the presence of an applied voltage in order to maximize the transmittance in white display state. This is because the transmittance of a light beam passing through a birefringent medium interposed between the cross-Nicole polarizers is proportional to sin2 (2α) where α (unit: rad) is an angle made by the axis of the polarizer and a slow axis of the birefringent medium. In typical MVA LCD devices, the LC molecules are aligned separately in four domains, or at azimuths of 45°, 135°, 225°, and 315°. Also in the four-domain VA LCD devices, LC molecules are often aligned in Schlieren pattern or in undesired directions near at a domain boundary or near the alignment control member. This is one factor causing loss of transmittance.
In view of these circumstances, circularly-polarizing plate-including VA LCD devices are provided as disclosed in Patent Document 1, for example. According to the LCD device, the transmittance of a light beam passing through a birefringent medium interposed between a right-circularly-polarizing plate and a left-circularly-polarizing plate orthogonal to each other is independent on an angle made by the axis of the polarizer and the slow axis of the birefringent medium. Therefore, a desired transmittance can be secured as long as the alignment of the LC molecules can be controlled, even if the alignment azimuth is not 45°, 135°, 225°, and 315°. Accordingly, a conical protrusion may be disposed at the center of a pixel, thereby aligning the LC molecules at every azimuth, or alternatively the LC molecules may be aligned at random azimuths without any control of the alignment azimuth, for example. In the present description, the VA LCD devices including circularly-polarizing plates are referred to as CPVA LCD devices or CP LCD devices. In addition, VA LCD devices including linearly-polarizing plates are referred to as LPVA LCD devices or LP LCD devices. As is well known, the circularly-polarizing plate is typically composed of a combination with a linearly-polarizing plate or a quarter-wave plate.
The circularly-polarized light beam switches its handedness when being reflected on a mirror and the like, and so when it enters a left-handed circularly-polarizing plate disposed on a mirror, the light beam that has been converted into a left-handed circularly-polarized light beam by the polarizing plate is converted into a right-handed circularly-polarized light beam by being reflected by the mirror. The right-handed circularly-polarized light beam can not transmit the left-handed circularly-polarizing plate. Thus, the circularly-polarizing plates are known to have an anti-reflection function. The anti-reflection function of the circularly-polarizing plates allows prevention of unnecessary reflection when display devices are viewed in bright environments such as outdoors. Therefore, the circularly-polarizing plate is known to have an effect of improving contrast ratio of display devices such as VA LCD devices in bright environments. The “unnecessary reflection” is considered to occur mainly due to transparent electrodes or metal wirings of TFT elements inside the display devices. If this unnecessary reflection occurs, even in a display device that can display an almost completely black screen in dark environments, the contrast ratio is lowered because the light amount in a black screen is increased under observation in bright environments.
As mentioned above, in CPVA LCD devices, the transmittance-improving effect and unnecessary reflection-preventing effect can be obtained, but common CPVA LCD devices have a low contrast ratio and can not show sufficient viewing angle characteristics as viewed from oblique directions. In this point, the CPVA LCD devices have room for improvement. In view of this, technologies involving use of birefringent layers (retardation films) for improving the viewing angle characteristics have been proposed. For example, Patent Document 1 discloses the following method (A); Patent Document 2 discloses the following method (B); Patent Document 3 discloses the following method (C); and Non-patent Document 1 discloses the following method (D).    (A) Use of two quarter-wave plates satisfying nx>ny>nz    (B) Combination use of two quarter-wave plates satisfying nx>nz>ny and a birefringent layer (III) satisfying nx=ny>nz    (C) Combination use of one or two half-wave plates satisfying nx>nz>ny in addition to the configuration (B)    (D) Combination use of two uniaxial quarter-wave plates (so-called A plates satisfying nx>ny=nz), a birefringent layer (III) satisfying nx=ny>nz, and a birefringent layer satisfying nx>nz>ny.[Patent Document 1]
Japanese Kokai Publication No. 2002-40428
[Patent Document 2]
Japanese Kokai Publication No. 2003-207782
[Patent Document 3]
Japanese Kokai Publication No. 2003-186017
[Non-patent Document 1]
Zhibing Ge and six others, “Wide-View Circular Polarizers for Mobile Liquid Crystal Displays”, IDRC08, 2008, p. 266-268